In Milikan's oil drop experiment,what is the terminal speed of an uncharged drop of radius $2\times 10^{-5}m$ and density $1.2\times 10^3Kgm^{-3}$?Take the viscocity of air at the temperature of the experiment is $1.8\times 10^{-5}pas$.How much is the viscous force on the drop at the speed?Neglect byoyancy of the drop due to air.